Lithographic printing plates can be made from photosensitive printing elements by imagewise exposing the element, thereby effecting a change in solubility between the exposed and unexposed areas, and then removing the more soluble areas in a development step. The development step can be either an additive or a subtractive process. In an additive process, the developer removes the more soluble areas, but at the same time it deposits an oleophilic reinforcing resin on the less soluble areas which remain on the plate. In a subtractive process, the non-image areas are removed by the developer, but there is no additional reinforcing resin. Printing elements which are developed by a subtractive process (i.e., subtractive elements) are generally based on light-induced polymerization, either free-radical or cationic.
Developers for subtractive printing elements can be emulsions or single-phase systems, either all-organic or aqueous-based. Emulsion developers have a tendency to redeposit polymer removed from non-image areas onto image areas, resulting in undesired specks on the developed plate. The emulsion also has a tendency to separate on storage, limiting its useful lifetime. Some emulsion developers exhibit high viscosity levels unsuitable for many lithographic plate processing machines.
All-organic developers are generally less desirable because of unpleasant odors, toxicity, flammability and serious disposal problems.
Therefore, there is a need for a single-phase aqueous-based developer for subtractive photosensitive printing elements.